1. Field of the Invention
This invention relates, generally, to safety devices. More particularly, it relates to a safety device that sounds an alarm when an object enters into a swimming pool at an unexpected time.
2. Description of the Prior Art
Many inventors have developed alarm means that are sounded when a child falls into a swimming pool at an unexpected time, i.e., when the alarm means is armed. Examples of patents on such devices include U.S. Pat. Nos. 5,177,691 to Welles et. al., 5,049,859 to Arnell, and 4,349,897 to Boehme et. al.
Some of these earlier devices incorporate sonar technology. As is well known, a sonar, lidar, or radar device transmits an ultrasonic or electromagnetic signal under water and analyzes the characteristics of the signals that return to the source after having been reflected back to the source by a target object. In a swimming pool setting, such a device is calibrated when the pool is empty and the wind is not blowing. The resulting reflected signals become the reference signals against which all other signals are compared. Thus, if a reflected signal returns to its source earlier than expected, it is presumed that it reflected off a foreign object and an alarm signal is generated and transmitted to an alarm device. Moreover, if a signal is received at a different frequency or out of phase with its original frequency or phase, reflection off a moving foreign object is presumed an alarm signal is generated.
More specifically, when an object such as a child falls or otherwise enters into a pool equipped with a sonar, lidar, or radar device, the frequency of the return signal is changed by the Doppler effect, thereby causing the device to generate an alarm signal. An ultrasonic or electromagnetic wave bouncing off such a foreign object will also return to the receive device at an earlier time than it would have but for the presence of the foreign object which shortened its path of travel around the pool.
The two primary sources of false alarms are wave action caused by wind, and self interference resulting from multi-path propagation.
Unfortunately,wind-activated wave action also causes the molecules of water in the pool to move toward or away from the receive device, thereby creating a Doppler effect and triggering the alarm.
As mentioned earlier, self interference is a problem, i.e., reflected waves often form interference patterns and cancel out each other; moreover, wave action alters the interference patterns. The receiving device compares the returning ultrasonic or electromagnetic waves with the reference ultrasonic or electromagnetic waves, notes that some of the returning waves are missing, as if absorbed by a target object, and activates an alarm.
After a pool owner has been repeatedly summoned to a pool just because the wind is blowing, or because an interference pattern has formed, he or she learns to ignore the alarm. Many people simply disconnect the alarm system after being summoned to the pool by numerous false alarms.
Accordingly, there is a need for a system that is not subject to false activation by wind-driven waves and by self interference.
However, at the time the present invention was made, it was not obvious to those of ordinary skill in this art how a system not subject to false activation could be developed.